This invention relates to a base station apparatus and mobile communication system. More particularly, the invention relates to a base station apparatus and mobile communication system for dealing with loss of calls or decline quality as caused by a decrease in transmission speed, wherein when a required transmission speed for a certain terminal or service is no longer met or the required transmission speed is met but without enough margin, the terminal is handed over to another frequency being used at the same position (location) without the terminal moving, thereby solving problems such as loss of calls.
Although a W-CDMA system is described below by way of example, the invention is capable of being implemented in mobile communication systems unless stated otherwise. That is, the present invention is applicable to mobile communications as a whole and not just to W-CDMA systems alone.
In a W-CDMA system, service areas (hexagonal areas) in a cellular configuration of the kind shown in FIG. 48 are formed, a radio base station is deployed at the center and each service area is composed of one or a plurality of sectors. FIG. 48 illustrates a three-sector arrangement. A radio base station will be referred to simply as a base station or Node B below.
Further, a plurality of frequencies (carrier waves) are assigned to each sector. FIG. 49 illustrates a case where two frequencies have been assigned to one sector. The frequency of a base station Node B1 in FIG. 49 is assumed to be f1 and its service area is indicated by the solid line. The frequency of a base station Node B2 is assumed to be f2 and its service area is indicated by the dashed line. For the sake of illustration, the solid and dashed lines are drawn off set from each other although they may just as well overlap. In FIG. 49, one base station, i.e., one Node B, is indicated for one frequency. However, one Node B may be associated with two frequencies, as illustrated in FIG. 50. In this specification, basically one transceiver TRX is shown to be deployed per frequency. Accordingly, there are two cases, namely a case (FIG. 49) where one transceiver (one frequency) is provided for one Node B, and a case (FIG. 50) where a plurality of transceivers TRX1, TRX2 are provided for one Node B.
It should be noted that a terminal normally is capable of sending and receiving using all frequencies employed in a W-CDMA system.
Handover
An operation in which a terminal UE1 moves from a cell CL1 of a base station BTS1 to a cell CL2 of a base station BTS2 to thereby change the base station that is the connection destination, as illustrated in FIG. 51, is referred to generically as handover. Handover based upon movement is well known and, depending upon the particular method, is classified into soft handover, hard handover, different-frequency handover and cell change. Simply “handover” will be used below.
Handover in a system such as the conventional 3GPP Release 99 and PDC (Personal Digital Cellular) is implemented under the initiative of a Radio Network Controller (RNC), which is above the base station hierarchically. That is, the RNC manages movement of terminals and controls handover between cells, between sectors, between different frequencies and between different systems. Specific examples of control are designating a handover-destination base station, designating the establishment of a radio link to a handover-destination base station and designating the re-establishment of a radio link to a terminal UE. The base station (Node B) at this time executes processing in accordance with the designation made by the RNC. Further, ordinary handover is carried out as the terminal moves.
HSDPA (High Speed Downlink Packet Access)
In mobile communication such as W-CDMA, data communication is performed using packets. In the case of W-CDMA, specifications are being reviewed in the 3GPP (3rd Generation Partnership Project) and packet communication is being performed between radio base stations and terminals (mobile telephones, etc.) using protocols that have been decided by the project.
In 3GPP at the present time, the HSDPA (High Speed Downlink Packet Access) scheme is being studied in order to perform packet communication at higher speeds. This is a technique for the purpose of adopting a high speed of 2 Mbps for packet communication on the downlink (communication from the base station to the terminal). As mentioned above, HSDPA is being studied with a view to implementing standardization in Release 5, which is a 3GPP specification. The major changes in HSDPA in comparison with 3GPP Release 9 of the conventional specifications are the composition of the radio channels, retransmission control and the introduction of a scheduler. The composition of radio channels will be described below in simple terms and a scheduler that is directly related to the present invention will be described as well.
FIG. 52 is a schematic view of the configuration of an HSDPA system. A radio access system in 3GPP comprises an RNC (Radio Network Controller) 1, a Node B (base station) 2 and UEs (User Equipment: terminals) 3. The RNC is connected to a CN (Core Network) 4.
With HSDPA, a {circle around (1)} HS-DSCH (High Speed-Downlink Shared Channel) in a wired downlink section and a {circle around (2)} HS-PDSCH (High Speed-Physical Downlink Shared Channel) in a wireless downlink section are used as packet-data transmission channels CH. That is, HS-DSCH and HS-PDSCH are channels exclusively for the downlink and are shared by a plurality of UEs. They transmit packets that have been encoded as by turbo encoding.
In a wireless downlink section, a {circle around (3)} HS-SCCH (High Speed Shared Control Channel) is set up as a high-speed control channel, and control information for allowing the plurality of UEs to receive packet data on the HS-PDSCH is transmitted. The control information includes a user identifier (UEID: User Equipment Identifier) and various parameters (radio spreading code, modulation scheme, data-length information, etc.) for receiving data on the HS-PDSCH. The HS-SCCH is shared by a plurality of the UEs.
Furthermore, in a wireless uplink section, a {circle around (4)} HS-DPCCH (High Speed Dedicated Physical Control Channel) is set up on a per-user basis. HS-DPCCH is a dedicated channel. This is a channel that transmits a value, which indicates the number of receivable bits, from each terminal to the base station based upon reception conditions (whether or not a packet could be received without errors) and the receiving state (the C/I, as one simple example). Notification indicative of the former, namely the reception conditions, is referred to as ACK (notification of acknowledgement of reception) or NACK (notification of reception failure), and information indicating the latter, namely the receiving state, is referred to as CQI (Channel Quality Indicator).
Channels in addition to those mentioned above are {circle around (5)} DL Associated DPCH (Downlink Associated Dedicated Physical Channel) and {circle around (6)} UL Associated DPCH (Uplink Associated Dedicated Physical Channel). These channels are radio channels established individually between each terminal and the base station. These are channels used in association with the HS-PDSCH in particular among the DPCHs (Dedicated Physical Channels) employed in conventional Release 99. These channels will be abbreviated to DL A-DPCH and UL A-DPCH below.
ACK/NACK and Retransmission Control
With HSDPA, data retransmission control is exercised between the Node B 2 and UE3. The UE3 reports ACK (notification of acknowledgement of reception) or NACK (notification of reception failure) with respect to the received data to the Node B 2 using the HS-DPCCH.
The flow of retransmission control is illustrated in FIG. 53, the structure of the terminal UE in FIG. 54 and the structure of the base station Node B in FIG. 55.
The terminal UE3 receives a packet, which has been transmitted by the above-mentioned HS-PDSCH, using a radio unit 3a, demodulates and decodes the packet using a demodulator 3b, performs a CRC check using a retransmission controller 3c and verifies the packet reception conditions (e.g., whether or not the packet could be received without errors). For example, in the event that no errors have been found, the terminal transmits ACK via a modulator 3d and radio unit 3e using the above-mentioned UL HS-DPCCH, thereby requesting the base station Node B to perform a new transmission. On the other hand, if an error is found as a result of the CRC check, the terminal transmits NACK using the UL HS-DPCCH, thereby requesting the base station Node B to perform retransmission. Retransmission is performed until error-free reception can be achieved, by way of example.
Meanwhile, the base station Node B receives UL HS-DPCCH by a radio unit 2a and demodulates and decodes the packet in a demodulator 2b. The base station then extracts the ACK/NACK signal in an ACK/NACK extraction unit 2c and performs retransmission control using a retransmission controller 2d. Specifically, in case of ACK, the retransmission controller 2d deletes a successfully transmitted packet that has been stored in a transmit buffer 2e. In case of NACK, the retransmission controller 2d retransmits an unsuccessfully transmitted packet, which has been stored in the transmit buffer 2e, via a modulator 2f and radio unit 2g using HS-PDSCH. Such retransmission control is carried out by a scheduler, which is described next.
Whether ACK or NACK is sent back changes depending upon whether or not receive data contains an error, which depends upon the receiving state of the terminal UE. As for the cause, factors that depend upon the states of C/I or/and S/N or/and the traveling speed of the terminal are significant. Here C/I stands for Carrier/Interference and corresponds to S/N and SIR (Signal/Interference), C represents signal power and I interference power with respect to interference. This is an index of the magnitude of interference. It indicates that the smaller the C/I, i.e., the greater the amount of interference, the greater the degradation of the receiving state.
Scheduler
In HSDPA introduced by 3GPP Release 5, the above-mentioned radio channels and a scheduler function that is for deciding the order of packet transmission are added anew. In order to describe the scheduler, a description will also be rendered with regard to HS-PDSCH. Unlike the conventional DPCH, HS-PDSCH is not a radio channel provided individually for a terminal that is a communicating party. That is, one HS-PDSCH, for example, is time-division multiplexed and is used by one terminal or by being shared by a plurality of terminals.
FIGS. 56(A) to 56(D) are diagrams useful in describing a mechanism for receiving packet data on the HS-PDSCH.
As shown at (A) of FIG. 56, a transmit cycle referred to as a “TTI” (Transmission Time Interval=2 ms) is set up on HS-SCCH. Control information is transmitted in conformity with the TTI and received by a plurality of UEs (two UEs #0 and #1) only if control information to be transmitted exists. The data transmitted on the HS-SCCH includes a user identifier (UEID: User Equipment Identifier) and various parameters (radio spreading code, modulation scheme, data-length information, etc.) for receiving data on the HS-PDSCH.
UE receives HS-SCCH data in all TTIs. For example, in slot #1 at (B) of FIG. 56, UE #1 and UE #2 receive the HS-SCCH data simultaneously. Each UE refers to the UEID in the data and compares it with its own ID. In this case, the UEID of the HS-SCCH data in slot #1 is “UE #1”, and therefore UE #0 discards the receive HS-SCCH data and UE #1 loads the control data contained in the receive HS-SCCH data. UE #1 thenceforth extracts a parameter, which is for HS-PDSCH receive, from the control data portion, and receives the packet data on the HS-PDSCH [(C), (D) of FIG. 56].
Upon receiving data, the UE #1 refers to a “sequence number” contained in the data and checks to determine whether there is loss of data. In a case where all data could be received without error (without CRC error) and without loss of data, the UE #1 reports ACK to Node B using the HS-DPCCH. Further, if data has been lost or a CRC check error has occurred, then the UE #1 reports NACK to Node B using the HS-DPCCH. Operation is similar with regard to slots #2 to #5 and slots #7 and #8. The UE #1 receives packet data via the HS-PDSCH of slots #1, #4, and the UE #0 receives packet data via the HS-PDSCH of slots #2 and #3, slot #5 and slots #7 and #8.
A scheduler executes scheduling management and retransmission control for deciding in which slot a packet should be transmitted and which terminal should be assigned. FIG. 57 is a diagram illustrating the structure of base station Node B that includes a scheduler. Here reference characters 2h represent a scheduler, 2i a handover controller, and 2j a CQI extraction unit for extracting CQI information, which is information indicating the receiving state of a terminal, from receive data.
An example of operation of the scheduler 2h will be described below. Depending upon the CQI reported from a terminal and the communication service content (quality of service QoS) of the data transmitted, the scheduler 2h decides the order of data transmission suited for each terminal and effects the transmission in this order. A specific example of the decision of an order will be given below. It should be noted that what follows is a representative method and that the method is not limitative in any way.
{circle around (1)} C/I Method
On the basis of the C/I, transmission is performed in order of descending C/I excellence. In case of HSDPA, a CQI of high value is adopted as an excellent C/I. There is a possibility that a terminal with a poor C/I will not be given an opportunity to transmit.
{circle around (2)} Round Robin Method
This is a method in which transmission is performed equally irrespective of the receiving state of the terminal.
{circle around (3)} Proportional Fairness Method
This is a method in which transmission time is equalized and transmission is performed in order of descending C/I excellence.
Further, in addition to the methods cited above, weighting with regard to the traffic class (Streaming class, Conversational class, Interactive class and Background class), which will be described later, also is conceivable. These traffic classes are referred to generically as QoS (Quality of Service). Maximum speed (bit/sec) and minimum speed (bit/sec), etc., are defined as parameters in QoS. In particular, in case of the Conversational or Streaming class, quick response is sought in view of the applications of these classes and the stipulation on minimum speed is severe. In cases where the minimum speed is not complied with, service may no longer be provided, service may be suspended and the quality of transmitted data may not be maintained. As examples that may readily be understood, frame advance may occur when a moving picture is transmitted, and audio or video may be interrupted.
{circle around (1)} Conversational class: This is a class in which a small-delay quality is required in both directions (example: voice).
{circle around (2)} Streaming class: This is a class in which a small-delay streaming service is required in one direction (example: distribution of real-time moving pictures).
{circle around (3)} Interactive class: This is a class that requires a response within a fixed period of time as well as a low error rate (example: a Web browser or server access).
{circle around (4)} Background class: This is a best-effort class of the kind that is implemented in the background (example: E-mail or ftp).
Problems of the Prior Art
Depending upon the propagation environment in which a terminal is placed or the traveling speed of the terminal, problems may arise. That is, the stipulation on minimum speed in the QoS of the transmitted service may not be adhered to, a call may be lost in the midst of communication and quality may decline. This will be described below using a specific example.
Assume that a certain terminal UE2 is receiving a service (transmission of a moving picture) that requires quick response, and that the throughput required is 2 Mbyte/sec. Assume that since the propagation environment has worsened and interference has increased (i.e., that C/I has deteriorated), it is necessary to repeat retransmission and the actual throughput (transmission speed) has become 1 Mbyte/sec. A problem which arises at this time is that frames of the moving picture are lost, motion of people, etc., becomes stiff and the moving picture becomes a still picture. In some cases service must be halted because the quality of the moving picture is not maintained.
Prior art (JP01-274524A) in which the communication rate (throughput) of a mobile terminal is measured to render a decision as to whether handover should be performed is available as art for preventing loss of calls and a decline in quality. However, this is not art in which a terminal having little margin is allowed to handed over to a base station of a different frequency.
Prior art (JP07-240959A) in which handover from a terminal having little margin is performed based upon the reception level is available as art for preventing loss of calls and a decline in quality. However, this is not art in which whether handover will be performed or not is decided based upon the communication rate (throughput) of a mobile terminal or delay time or transmission power, and handover is allowed to be made to a base station of a different frequency.
Prior art (JP10-136425A) in which handover is performed at a different frequency is available. However, this is not art in which handover is performed upon detecting a terminal that is likely to experience loss of a call or a decline in quality based upon the communication rate (throughput) of the mobile terminal, delay time or transmission power.